The present invention relates generally to automatic locking mechanisms for power driven shafts. The invention is particularly well-suited for application in power tools, especially those of the hand-held variety used for driving threaded fasteners into a workpiece, for example.
Power tools such as power screwdrivers, nut drivers, and other such fastener drivers have become widely used for power-driving threaded fasteners into a workpiece or for driving one threaded fastener onto or into another threaded fastener. Sometimes, though, due to the size, length, or condition of the threaded fastener, such power driving tools lack sufficient torque to tighten (or loosen) the threaded fasteners to the full extent desired by the operator. In such instances, operators frequently use the power driving tool in a de-energized state or in a locked-armature condition to forcibly manually tighten the fastener. Also, in some cases, operators use the tool to manually set a fastener in order to more precisely control the final amount of torque applied to the fastener.
While such manual torque-applying usages are well known and common, they can sometimes result in damage to the power tool in the form of bent or broken internal drive components or even possible electrical damage to the power tool's motor. In addition, if the operator uses the power tool to manually tighten a fastener when the motor is de-energized, the back-applied torque can cause slippage in various drive components or can otherwise be less than fully effectual to allow the operator to manually tighten (or loosen) the fastener.
Accordingly, various shaft lock mechanisms and designs have been provided in hand-held power tools to alleviate these problems or to aid in manual torque-applying operations. One example of which, wherein the shaft lock mechanism is on the power tool's output shaft, is shown in U.S. Pat. No. 5,016,501, granted to Holzer in 1991. However, many of these mechanisms have themselves proved disadvantageous in that large or excessive amount of manually-applied back-torque can damage or break the shaft lock mechanisms themselves. The present invention, therefore, seeks to provide an automatic shaft lock mechanism that substantially prevents the transmission of back-torque during manual tightening operations in ways that could result in component breakage, motor damage, or slippage. The present invention also seeks to provide such a shaft lock mechanism that is not located at the tool's output shaft and that can thus take advantage of the tool's output gearing and thus be sturdier and more effective.
In accordance with one preferred example of the present invention, an interlock mechanism including an automatic shaft lock apparatus is provided for a power tool having a housing, a longitudinally-extending or axially-extending rotatable armature shaft enmeshed (either itself or through a pinion gear) with an intermediate gear disposed within the housing for rotation in response to rotation of the armature shaft, and an intermediate shaft disposed in the housing for rotation therein and rotationally interconnected with the tool's bit-holding chuck. The interlock mechanism drivingly and rotationally interconnects the intermediate shaft with the intermediate gear in order to cause bidirectional "forward-torque" rotation of the intermediate shaft in one of two directions in response to corresponding rotation of the intermediate gear, while the automatic shaft lock portion of the interlock mechanism prevents rotation of the intermediate gear in response to an external rotational force imposed on the intermediate shaft in a second opposite or "back-torque" direction.
In order to accomplish this, the automatic shaft lock includes a hollow cylindrical cavity formed in a fixed portion of the housing, preferably in the form of a hollow cylindrical cavity (with or without an internal wear sleeve) carried by a fixed bearing plate in the housing, with the hollow cylindrical cavity being radially offset relative to the armature shaft and having a cylindrical interior cavity surface therein. At least one drive lug (and preferably more than one) is fixedly disposed on the intermediate gear for concentric rotation therewith and extends longitudinally or axially into the hollow cylindrical cavity at the radial periphery thereof, with each of the preferred drive lugs having a drive projection extending radially inwardly.
An anvil is fixedly disposed (such as by press-fit, for example) on the intermediate shaft for concentric rotation therewith and is disposed within the hollow cylindrical cavity.
The anvil has an external diameter smaller than the diameter of the interior cavity surface of the hollow cylindrical cavity and has at least one, and preferably more than one, longitudinally-extending anvil channels recessed radially inwardly therein for interlockingly receiving the radially inwardly extending drive projections therein in a driving relationship therewith. The anvil channels have a circumferential width greater than the circumferential width of the drive projection in order to permit a predetermined amount of limited relative rotation therebetween. The anvil, adjacent drive lugs, and the interior cavity surface of the hollow cylindrical cavity together form a chamber within the cylindrical cavity, within which at least one longitudinally-extending cylindrical locking pin is disposed, resting between the anvil and the interior cavity surface of the hollow cylindrical cavity and between circumferential sides of the adjacent drive lugs.
The preferred anvil has a radially inwardly recessed flat portion between each of the adjacent pairs of anvil channels such that there is more radial clearance for the locking pin (between the interior cavity surface and the anvil) at a generally intermediate location of the chamber (between the anvil channels) than there is at the circumferential ends of the chamber, closely adjacent the anvil channels, where the pin or pins become radially "pinched" between radially outwardly-raised portions or radially outwardly-protruding "bosses" on either circumferential side of each of the anvil channels. The locking pins and the anvil are free to rotate in response to interlocking rotation of the drive lugs, the intermediate gear, and the anvil, in response to forward-torque rotation of the intermediate gear being driven (in either rotational direction) by rotation of the armature shaft, with the locking pins being urged and engaged by circumferential sides of the drive lugs to maintain them in the radially relatively unrestricted area defined by the above-mentioned flat anvil portions and the cavity inner surface. The pins, however, become radially wedged or pinched between the anvil boss surfaces closely adjacent the channels and the interior cavity surface in response to an externally-applied rotational back-force or back-torque imposed on the intermediate shaft when the intermediate gear and the armature shaft are rotationally stationary, or in response to such back-torque imposed in an opposite direction from the direction of the rotational force on the intermediate shaft imposed by the armature shaft, the intermediate gear, the drive lug, and the anvil when such external rotational back-torque is being imposed on the intermediate shaft of an energized power tool. In either case, the automatic shaft lock prevents transmission of the external rotational back-force and consequent back-torque from being imposed from the intermediate shaft and the anvil to the intermediate gear and the armature shaft. The automatic shaft lock of the present invention functions equally in either rotational direction.
It should be emphasized that such back-torque imposed on the tool's output shaft is reduced by virtue of being transmitted through the relatively large output gear and the relatively small output pinion gear before it is transmitted to the shaft lock mechanism. Or, stated another way, this arrangement allows the shaft lock mechanism to resist such back-torque with a "torque-amplified" resistance. This protects the shaft lock mechanism from breakage, as well as locating it more internally (at a position in the drive train that is internally-located relative to the output shaft) where it is better protected from dust or other external contaminants.
Additional objects, advantages, and features of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings.